1. Field of the Invention
The present invention concerns a method of monitoring the cooking temperature of meat patties. More specifically the present invention concerns a method of monitoring-the cooking temperature of meat patties by determining the concentration of coloured compounds in meat juice.
2. Description of the Related Art
In the US, rules for controlling meat have been strengthened since the incident with bacterial contamination of meat was discovered in a Jack in the Box restaurant. Food Safety and Inspection Service (FSIS; equivalent to FDA) has in 1993 sent out directives which should be followed and inspections are carried out by local health units Since 1996 similar directives in Europe prescribe that each food processing company should work in a preventive way. Listeria monocytogenes are the most serious contaminating bacteria but also Salmonella, E.coli O157:H7 and Staphylococcus aureus are of interest to monitor in all types of food. Since the first incident on bacterial contamination (Jack in the Box), most fast-food chains like McDonalds, Burger King and Wendy's over-cook their hamburgers for safety reasons.
Different methods to determine if patties have been well cooked have been used. FSIS recommends thermocouple thermometers inserted in the patties to verify that a certain temperature has been reached. In Europe meat samples are generally sent to external labs for bacterial analysis. Both temperature and cooking time are of importance for killing bacteria.
Measurements of correct temperature of cooked patties are difficult mainly due to the heat transfer between parts/spots of the patties during the cooking process. A better indicator of meat temperature would be a testing of a meat juice compound, flowing in the pores of the meat during cooking. Myoglobin and catalase (an enzyme) have been suggested for monitoring meat juice temperature.
Myoglobin is the major pigment responsible for fresh meat colour. In fresh meat, the pigment can exist in three different forms: The reduced form of myoglobin is purplish, the oxygenated form is bright red and the oxidised form is brown. Fresh meat colour is determined by the relative abundance of these three forms. The consumer often associates the natural bright red colour to the freshness of the product, since brown colour of the oxidised pigment results from prolonged storage. Myoglobin is found in the muscle and is very similar in structure to haemoglobin. The difference in structure is the number of polypeptide chains. It is fairly insensitive to a change in pH. This is not a major problem with meat since the pH is fairly constant (pH 5.3-5.5). It denatures at temperatures above 150.degree. F. (65.5.degree. C.) and might thus be of interest as a temperature indicator in processing of bovine meat (hamburgers).
One method for checking that meat has been heated to a temperature of about 80.degree. C. is disclosed in an article "Colour Determination of beef juices as an indication of beef cooking temperatures" by Nusimovich, A. D. et al in Meat Science 1979 3(3) 233-244. This method is based on the measurement of the optical density at 10 nm intervals over the wave length range 380-770 nm. It is obvious that this method is comparatively labour-intensive and therefore inadequate for industrial application. Additionally, this method is suggested for checking that a specific virus, the so called foot-and-mouth virus has been eliminated. It is furthermore stated that in this known method the error of prediction is unacceptable large.